Interpretive Summary: Huanglongbing (HLB) is a bacterial disease of citrus which is causing substantial economic losses to the citrus industry worldwide. No true resistance to HLB has been reported in commercial citrus varieties and sweet oranges, which account for most of the varieties grown in Florida, are very susceptible to the disease. Rootstock is an important component of commercial citrus production, and tolerance to HLB has been reported for some rootstock varieties, particularly trifoliate orange and some of its hybrids. The objective of our study was to investigate if rootstock selection has an effect on HLB disease development under natural conditions in the field. Four field trials with sweet orange scion on 15 different rootstocks were evaluated for HLB incidence, foliar disease symptoms, canopy damage, and stem growth during the first years after infection with the suspected causal agent of HLB. Trials ranged from two to nine years in age and included hybrids of trifoliate orange along with other rootstocks standard for citrus production in Florida. Fruit yield and fruit quality were analyzed for the oldest trial. Our study showed that rootstock did not affect disease incidence and that trees on all rootstocks were considerably damaged by HLB. However, disease effects were less detrimental in trees grafted on some of the rootstock selections depending on the trial. Tolerance to HLB was generally higher in the oldest trees. It is suggested that high vigor-inducing rootstock varieties may enable younger trees to outlast the damaging effects of the disease.

Technical Abstract:
Huanglongbing (HLB), a bacterial disease of citrus, is causing substantial economic losses to the citrus industry worldwide. Sweet oranges are highly susceptible to the disease, and account for nearly 90% of all varieties grown in Florida. Rootstock is an important component of commercial citrus production, and tolerance to HLB has been reported for some rootstock varieties. Our objective was to investigate if rootstock selection has an effect on HLB disease development under natural conditions in the field. Four field trials with sweet orange scion on 15 different rootstocks were evaluated for incidence of Candidatus Liberibacter asiaticus (Las), the suspected causal agent of HLB, as well as incidence of foliar disease symptoms, canopy damage, and stem growth during the first years after Las began to spread into the trials. Trials ranged from two to nine years in age and included hybrids of trifoliate orange along with other rootstocks standard for citrus production in Florida. Fruit yield and soluble solids content were analyzed for the oldest trial. Our study showed that rootstock did not affect disease incidence and that trees on all rootstocks were considerably damaged by HLB. However, tolerance to HLB was higher in trees grafted on some rootstock selections. In the youngest trial, stem diameters on Volkamer lemon increased 53% while trees on US-852, Benton citrange and Swingle citrumelo grew the least at 21-26% from 2008-2010. Depending on trial and time of observation, foliar HLB symptoms on US-897 were less than on many other rootstocks, with ranks of 3.3-4.0 compared with 4.5-4.9 observed for US-812, US-852, Carrizo citrange and Kinkoji. Canopy damage ranks were 1.4 to 2.6 for trees on US-802 compared with 3.9-4.1 for trees on US-812, U-852, Sour orange, and Swingle in the oldest trial. In the youngest trials, trees on Volkamer showed least canopy damage while trees on US-852 and Benton were more affected. Highest fruit yields of 30-64 kg/tree were obtained from trees on US-802 and Carrizo. In 2010, highest juice soluble solids content of 2.8-2.9 kg was observed for trees on US-897 and US-812.